Washington saw the New York attack as potentially deciding American independence.

Evidence

A British occupation of the corridor between Montreal and New York City would divide New England from the other states and could doom the patriot cause. This prospect explained the dismay that Washington expressed in July regarding the retreat of American forces to Ticonderoga and his observation in August that the pending British attack on New York “will Probably decide the Fate of America.” — Edward J. Larson, Declaring Independence, 237